JP7382795B2 - Electrolyte for secondary batteries and rocking chair type secondary batteries - Google Patents

Description

The present invention relates to an electrolytic solution for a secondary battery and a rocking chair type secondary battery containing the same.

BACKGROUND ART Conventionally, so-called rocking chair type secondary batteries have been developed in which charging and discharging are performed by ions moving back and forth in an electrolytic solution disposed between electrodes. In recent years, electrolytes with excellent oxidation stability and thermal stability have been required for next-generation rocking chair type secondary batteries. For example, we focused on increasing the concentration of an alkali metal salt as an electrolyte to improve charge/discharge performance, and solvating the alkali metal salt and a solvent to improve oxidation stability and thermal stability. Electrolytes with high alkali metal salt concentrations (ie, concentrated electrolytes) are attracting attention.

Glyme-based solvents (ether-based compounds with an alkyl group at the end) and carbonate-based solvents (carbonate-based compounds with an alkyl group at the end) are the mainstream solvents for concentrated electrolytes and have been widely studied. For example, in Patent Document 1, a rechargeable lithium battery made using 1,2-dimethoxyethane (methyl monoglyme), ethylene carbonate, ethyl methyl carbonate, etc. as a solvent for a concentrated electrolyte has a high energy density and an improved cycle life. It is stated that the lifespan has been achieved.

Although the lithium battery described in Patent Document 1 has improved charging and discharging performance due to its high alkali metal salt concentration, the viscosity of its electrolyte tends to increase. In general, in high-viscosity electrolytes, the movement of alkali metal ions derived from alkali metal salts is restricted, so if the salt concentration is the same, electrolytes with lower viscosity improve charge and discharge due to improved ionic conductivity. Improved performance.

Patent Document 2 discloses that methyl monoglyme (G1), methyl diglyme (G2) and lithium salts are used for the purpose of providing an electrolyte for lithium ion secondary batteries that provides battery characteristics such as excellent cycle characteristics and rate characteristics. A solvated ionic liquid has been proposed in which the ratio of the total number of moles of oxygen atoms contained in G1 and G2 to the number of moles of lithium is 3.0 or more and less than 4.0. In particular, when the above ratio is 4.0 or more, when lithium ions participate in electrode reactions, the solvent molecules freed by desolvation cause a decomposition reaction at the electrode interface, resulting in deterioration of the cycle characteristics of the secondary battery. It teaches what can be done.

Special table 2018-505538 publication JP 2017-139068 Publication

Further improvements in lithium ion secondary batteries are desired. That is, there is a need for a technology that achieves both generally contradictory properties such as ionic conductivity and redox stability at a high level. The present invention aims to newly propose such technology.

式（２）中、Ｒ^４およびＲ^５は、それぞれ独立して、炭素数１～８の直鎖状または分岐状のアルキル基を表し、Ａ^４は、炭素数２～４のアルキレン基を表し、ｎはオキシアルキレン基の平均付加モル数であって、１～５の整数を表す。
［５］前記アルカリ金属塩のアルカリ金属１モル当たり、前記エステル結合を含む含酸素溶媒と前記グライムとに含まれるエーテル酸素およびカルボニル酸素の合計が３モル以上、かつ、８モル以下である、態様１～４のいずれか一項に記載の二次電池用電解液。
［６］前記アルカリ金属塩と、前記エステル結合を含む含酸素溶媒と、前記グライムとを実質的に１：１：１のモル比で含む、態様１～５のいずれか一項に記載の二次電池用電解液。
［７］前記アルカリ金属塩が、リチウムビス（トリフルオロメタンスルホニル）イミド（LiTFSI）、リチウムビス（フルオロスルホニル）イミド（LiFSI）、テトラフルオロホウ酸リチウム（LiBF₄）、ヘキサフルオロリン酸リチウム（LiPF₆）、過塩素酸リチウム（LiClO₄）及びリチウムビス（ペンタフルオロエタンスルホニル）イミド（LiBETI）からなる群から選択される、態様１～６のいずれか一項に記載の二次電池用電解液。
［８］正極と、負極と、態様１～７のいずれか一項に記載の二次電池用電解液とを含んでなる、ロッキングチェア型二次電池。 As a result of intensive studies to solve the above problems, the present inventors found that by combining glyme with a specific oxygen-containing solvent, the ionic conductivity, rate characteristics, oxidation stability, reduction stability, thermal stability, etc. We have discovered that an electrolyte for secondary batteries can be obtained that achieves a high level of battery characteristics. That is, the present invention includes the following aspects.
[1] An electrolytic solution for a secondary battery comprising an alkali metal salt, an oxygen-containing solvent containing an ester bond, and glyme.
[2] The electrolytic solution for a secondary battery according to aspect 1, wherein the oxygen-containing solvent containing an ester bond is represented by the following general formula (1):
R ¹ -O-(A-O) _n -CH ₂ CH(R ³ )-COO-R ² (1)
In formula (1), R ¹ and R ² each independently represent a linear or branched alkyl group having 1 to 4 carbon atoms, R ³ represents a methyl group or H , and A each independently represents a carbon number It represents 2 to 4 alkylene groups, and n is the average number of added moles of oxyalkylene groups, and represents an integer of 0 to 5.
[3] The electrolytic solution for a secondary battery according to aspect 2, wherein in formula (1), both R ¹ and R ² are methyl groups, R ³ is H, and n is 0.
[4] The electrolytic solution for a secondary battery according to any one of aspects 1 to 3, wherein the grime is represented by the following formula (2):

In formula (2), R ⁴ and R ⁵ each independently represent a linear or branched alkyl group having 1 to 8 carbon atoms, and A ⁴ represents an alkylene group having 2 to 4 carbon atoms. , n is the average number of added moles of oxyalkylene groups, and represents an integer of 1 to 5.
[5] An embodiment in which the total amount of ether oxygen and carbonyl oxygen contained in the oxygen-containing solvent containing an ester bond and the glyme is 3 mol or more and 8 mol or less per 1 mol of the alkali metal of the alkali metal salt. 5. The electrolytic solution for secondary batteries according to any one of 1 to 4.
[6] The compound according to any one of aspects 1 to 5, comprising the alkali metal salt, the oxygen-containing solvent containing an ester bond, and the glyme in a molar ratio of substantially 1:1:1. Electrolyte for secondary batteries.
[7] The alkali metal salt is lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium tetrafluoroborate (LiBF ₄ ), lithium hexafluorophosphate (LiPF ₆ ). ), lithium perchlorate (LiClO ₄ ), and lithium bis(pentafluoroethanesulfonyl)imide (LiBETI), the electrolyte for a secondary battery according to any one of aspects 1 to 6.
[8] A rocking chair type secondary battery comprising a positive electrode, a negative electrode, and the electrolytic solution for a secondary battery according to any one of Aspects 1 to 7.

According to the present invention, an electrolytic solution for a secondary battery is provided that achieves high levels of battery characteristics such as ionic conductivity, rate characteristics, oxidation stability, reduction stability, and thermal stability.

It is a graph showing the oxidation stability of various electrolytes. It is a graph showing the reduction stability of various electrolytes. It is a graph showing rate characteristics of various electrolytes. 4 is a graph showing the rate characteristics of various electrolytic solutions following FIG. 3. It is a graph showing the oxidation stability of various electrolytes. It is a graph showing the reduction stability of various electrolytes. It is a graph showing rate characteristics of various electrolytes. 8 is a graph showing the rate characteristics of various electrolytic solutions following FIG. 7. It is a graph showing the oxidation stability of various electrolytes. It is a graph showing the reduction stability of various electrolytes. It is a graph showing rate characteristics of various electrolytes.

Hereinafter, exemplary embodiments of the present invention will be described, but the present invention is not limited to the following embodiments.
The present invention provides an electrolytic solution for a secondary battery comprising an alkali metal salt, an oxygen-containing solvent containing an ester bond, and glyme.

(alkali metal salt)
The alkali metal salt used in the electrolyte according to the present invention may be any alkali metal salt understood by those skilled in the art to be useful as an electrolyte for rocking chair secondary batteries. The alkali metal constituting the alkali metal salt can be selected from lithium, sodium, potassium, rubidium, and cesium, but from the viewpoint of ease of acquisition or production, lithium, sodium, or potassium is preferable. Furthermore, from the viewpoint of ease of complex formation with an oxygen-containing solvent containing an ester bond and glyme, lithium is more preferable. That is, the electrolytic solution in a more preferred embodiment is an electrolytic solution for a lithium ion secondary battery.

Counter anions constituting the alkali metal salt include Cl, Br, I, BF ₄ , PF ₆ , CF ₃ SO ₃ , SCN, ClO ₄ , CF ₃ CO ₂ , AsF ₆ , SbF ₆ , AlCl ₄ , N(CF ₃ SO ₂ ) ₂ , PF ₃ (C ₂ F ₅ ) ₃ , N(FSO ₂ ) ₂ , N(FSO ₂ )(CF ₃ SO ₂ ), N(C ₂ F ₅ SO ₂ ) ₂ , N(C ₂ Examples include F ₄ S ₂ O ₄ ), N(C ₃ F ₆ S ₂ O ₄ ), N(CN) ₂ , N(CF ₃ SO ₂ )(CF ₃ CO), C(CF ₃ SO ₂ ) ₃ , etc. can.

The lithium salt, which is a suitable example of the alkali metal salt, may be any lithium salt understood by those skilled in the art to be useful as an electrolyte for rocking chair type secondary batteries, such as LiTFSI, LiFSI, Examples include LiBF ₄ , LiPF ₆ , LiClO ₄ , LiBETI, LiAsF ₆ , LiCF ₃ SO ₃ , Li(CF ₃ SO ₂ ) ₃ C, LiI, LiSCN, and the like. Among the preferred lithium salts, LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), LiFSI (lithium bis(fluorosulfonyl)imide), LiBF ₄ (lithium tetrafluoroborate), LiPF ₆ (lithium hexafluorophosphate), Selected from the group consisting of lithium perchlorate (LiClO ₄ ) and LiBETI (lithium bis(pentafluoroethanesulfonyl)imide). These lithium salts are commercially available.

(Oxygen-containing solvent containing ester bond)
The oxygen-containing solvent containing an ester bond used in the electrolyte according to the present invention is a compound capable of forming a complex with the alkali metal. One embodiment of such a compound is represented by the following general formula (1).
R ¹ -O-(A-O) _n -CH ₂ CH(R ³ )-COO-R ² (1)
In formula (1), R ¹ and R ² each independently represent a linear or branched alkyl group having 1 to 4 carbon atoms, R ³ represents a methyl group or H , and A each independently represents a carbon number It represents 2 to 4 alkylene groups, and n is the average number of added moles of oxyalkylene groups, and represents an integer of 0 to 5.

R ¹ and R ² in the above general formula (1) are linear or branched alkyl groups having 1 to 4 carbon atoms, from the viewpoint of further improving the ability to form a complex with an alkali metal. Specific examples of such alkyl groups include methyl group, ethyl group, propyl group, isopropyl group, butyl group, isobutyl group, t-butyl group, etc. Among them, methyl group, ethyl group, and propyl group are preferable. , especially methyl group. Further, R ³ is preferably H 2 .

The total number of carbon atoms in R ¹ , R ² and R ³ in the above general formula (1) is preferably 5 or less, more preferably 3 or less, from the viewpoint of further increasing the electrolyte salt concentration while suppressing an increase in the viscosity of the electrolyte. be. Here, R ¹ and R ² may have a structure having a substituent, but in that case, the number of carbon atoms in each of R ¹ and R ² is the number including the substituent. Examples of substituents that R ¹ and R ² may have include an alkoxy group having 1 to 3 carbon atoms, a fluoro group, a chloro group, a bromo group, a cyano group, a nitro group, a hydroxyl group, an aldehyde group, a carboxyl group, etc. It will be done.

A in the above general formula (1) is preferably an ethylene group having 2 carbon atoms or a propylene group having 3 carbon atoms, particularly preferably an ethylene group, from the viewpoint of further improving the ability to form a complex with an alkali metal. . Further, n in the above general formula (1) is preferably 2 or less, more preferably 1 or less, and most preferably 0, from the viewpoint of further improving the ability to form a complex with an alkali metal.

Particularly preferred compounds as the oxygen-containing solvent containing an ester bond according to the present invention include methyl 3-methoxypropionate (MMP), ethyl 3-methoxypropionate, methyl 3-ethoxypropionate, and 3-methoxy-2-methylpropionate. Methyl, etc. These compounds can be used as appropriate commercially available products.

In the electrolytic solution according to the present invention, at least a portion, preferably substantially all, of the alkali metal derived from the alkali metal salt forms a complex with an oxygen-containing solvent containing an ester bond. Formation of such a complex improves the oxidative stability and thermal stability of the electrolyte of the secondary battery. The presence of the complex can be confirmed by thermogravimetry.

(Grime)
Glyme is a compound that can form a complex with the alkali metals mentioned above.
Glyme is a general term for (poly)alkylene glycol diethers having an oxyalkylene structure. Glyme can be used without particular limitation as long as it is a (poly)alkylene glycol diether having an oxyalkylene structure, but a compound represented by the following formula (2) is particularly preferred.

In formula (2), R ⁴ and R ⁵ each independently represent a linear or branched alkyl group having 1 to 8 carbon atoms. The alkyl group may be saturated or unsaturated, and may also be linear, branched, or cyclic. Examples of linear, branched, or cyclic saturated alkyl groups having 1 to 8 carbon atoms include methyl group, ethyl group, n-propyl group, isopropyl group, cyclopropyl group, n-butyl group, isobutyl group, sec-butyl group, tert-butyl group, cyclobutyl group, n-pentyl group, isopentyl group, neopentyl group, cyclopentyl group, n-hexyl group, cyclohexyl group, n-heptyl group, cycloheptyl group, n-octyl group, 2 -Ethylhexyl group, cyclooctyl group, etc. Examples of the linear, branched, or cyclic unsaturated alkyl group having 1 to 8 carbon atoms include vinyl group, allyl group, 2-methylallyl group, 3,3-dimethylallyl group, and 2,3,3 -Trimethylallyl group, 3-butenyl group, 1,3-butadienyl group, 4-pentenyl group, 2,4-pentadienyl group, 5-hexenyl group, 1,3,5-hexatrienyl group, 2-cyclopentenyl group , 2,4-cyclopentadienyl group, 2-cyclohexenyl group, 2,5-cyclohexadienyl group, and the like. Further, A ⁴ represents an alkylene group having 2 to 4 carbon atoms. Examples of the alkylene group having 2 to 6 carbon atoms include ethylene group, propylene group, methylmethylene group, 1-methylethylene group, butylene group, 1-methylpropylene group, 1,1-dimethylethylene group, 1,2- Examples include dimethylethylene group, pentylene group, propylethylene group, 1-ethyl-2-methylethylene group, hexylene group, and butylethylene group. Note that the plurality of A ⁴ 's may be one type of alkylene group, or two or more types of alkylene groups. When the polyoxyalkylene represented by (A ⁴ O) _n contains two or more types of alkylene groups, the addition state may be random or block-like. Further, n is the average number of added moles of oxyalkylene groups, and represents a real number from 2 to 20. Among these, n is preferably a real number from 2 to 10, more preferably from 2 to 5.

Examples of grime having such a structure include ethylene glycol dimethyl ether (G1), diethylene glycol dimethyl ether (G2), triethylene glycol dimethyl ether (G3), tetraethylene glycol dimethyl ether (G4), and pentaethylene glycol dimethyl ether (G5). ), hexaethylene glycol dimethyl ether (G6), heptaethylene glycol dimethyl ether (G7), octaethylene glycol dimethyl ether (G8), nonaethylene glycol dimethyl ether (G9), decaethylene glycol dimethyl ether (G10), tetraethylene glycol methyl butyl ether, diethylene glycol methyl Examples include benzyl ether, diethylene glycol methyl 2-ethylhexyl ether, diethylene glycol methylhexyl ether, and triethylene glycol butyl octyl ether. Such grime is particularly suitable for transporting lithium ions. These glymes can be used alone or in combination of two or more types.

In the electrolytic solution for a secondary battery according to the present invention, the total amount of ether oxygen and carbonyl oxygen contained in the oxygen-containing solvent containing an ester bond and the glyme is 3 moles or more (preferably) per 1 mole of the alkali metal of the alkali metal salt. is 4 mol or more) and 8 mol or less (preferably 7 mol or less). Here, the ether oxygen and carbonyl oxygen are oxygen atoms that provide an unpaired electron pair for the oxygen-containing solvent containing the ester bond and the glyme to coordinate to the alkali metal ion during complex formation. For example, in the case of methyl 3-methoxypropionate (MMP), which is an oxygen-containing solvent containing an ester bond, a total of two oxygen atoms are present, one ether oxygen constituting the methoxy group and one carbonyl oxygen constituting the ester group. Atoms contribute to coordination to alkali metal ions. In addition, ethylene glycol dimethyl ether (G1), diethylene glycol dimethyl ether (G2), triethylene glycol dimethyl ether (G3), and tetraethylene glycol dimethyl ether (G4), which are glymes, have 2, 3, 4, and 5 ether oxygen atoms, respectively. contributes to coordination to alkali metal ions. Therefore, in the case of an electrolytic solution combining 1 mole of MMP and 1 mole of G1 per mole of alkali metal, the total of ether oxygen and carbonyl oxygen is 4 moles. Further, in the case of an electrolytic solution in which 1 mol of MMP and 1 mol of G2 are combined per 1 mol of alkali metal, the total amount of ether oxygen and carbonyl oxygen is 5 mol. Furthermore, in the case of an electrolytic solution that combines 1 mol of MMP and 1 mol of G3 per mol of alkali metal, the total amount of ether oxygen and carbonyl oxygen is 6 mol. In this way, the number of oxygen atoms contributing to coordination to the alkali metal ion can be adjusted by changing the type of solvent to be combined. Similarly, the number of oxygen atoms contributing to coordination to the alkali metal ion can also be adjusted by the number of moles of the solvent species to be combined. For example, when adjusting the total of ether oxygen and carbonyl oxygen to 4 moles per mole of alkali metal, 1 mole of MMP and 0.5 mole of G3 may be combined. When the total amount of ether oxygen and carbonyl oxygen is within the range of 3 to 8 mol (preferably 4 to 7 mol) per mol of alkali metal ion, ionic conductivity, rate characteristics, oxidation stability, reduction stability, It is possible to achieve a well-balanced and high level of battery characteristics such as thermal stability.

The mixing molar ratio of the oxygen-containing solvent containing an ester bond and glyme is not particularly limited as long as the total of the ether oxygen and carbonyl oxygen per mole of the alkali metal is within the above-mentioned predetermined range, but is generally 1: The ratio may be in the range of 4 to 4:1, preferably 1:3 to 3:1, more preferably 1:2 to 2:1.

(Other ingredients)
In addition to the above-mentioned components, the electrolytic solution according to the present invention contains a viscosity modifier (e.g. hydrofluoroether), an antioxidant, a film forming agent, a preservative, a surfactant, a stabilizer, an ultraviolet absorber, an inorganic or organic It may further contain additives such as fillers and flame retardants.

The electrolytic solution according to the present invention can be produced by mixing the above-mentioned oxygen-containing solvent containing an ester bond, glyme, and alkali metal salt together with the above-mentioned other additives as necessary in a dry atmosphere such as in a normal dry room. Can be manufactured.

In a rocking chair type secondary battery, stability with respect to electrode potential (ie, potential window) is important. When the oxidation and reductive decomposition of an electrolyte is tested using a potential-controlled electrode, the potential (oxidation or reductive decomposition potential) at which the decomposition current (obtained as a function of the potential) is measured is determined as the potential window, and this is the potential window for the electrolytic solution. It is an indicator of the oxidative stability of the liquid. The potential window of the electrolytic solution is preferably 3V or more, more preferably 4V or more, still more preferably 5V or more. The above potential window is a value measured by linear sweep voltammetry.

(Rocking chair type secondary battery)
The present invention further provides a rocking chair type secondary battery having a positive electrode, a negative electrode, and an electrolyte according to the present invention. In a preferred embodiment, the rocking chair type secondary battery is a lithium ion secondary battery. There are no particular restrictions on the positive electrode and negative electrode, and active materials for the positive electrode include metal oxides such as lithium cobalt oxide, lithium manganate, lithium iron phosphate, lithium nickel manganese cobalt oxide (NMC), and lithium nickel cobalt aluminate (NCA). substance, elemental sulfur, lithium polysulfide (Li ₂ S _x (0<x≦8)), sulfur-based metal polysulfide (MS _x (M=Ni, Cu, Fe, 0<x≦2)), organic Sulfur compounds such as disulfide compounds and carbon sulfide compounds can be used as appropriate. In addition, active materials for negative electrodes include carbon-based materials such as graphite, hard carbon, and soft carbon, crystalline or amorphous silicon, silicon-containing compounds such as silicon oxide, silicone resin, and silicon-containing polymer compounds, and titanium. Lithium oxide, elemental lithium metal, LiC, LiSi, LiGr, LiAl, LiIn, etc. can be used. In one embodiment, a rocking chair type secondary battery has a separator placed between a positive electrode and a negative electrode. As the separator, various sheet-like materials capable of holding the electrolytic solution can be used, such as porous membranes, nonwoven fabrics, etc. made of materials such as polyolefin, polyimide, cellulose, and glass.

EXAMPLES Hereinafter, the present invention will be specifically explained with reference to examples, but the scope of the present invention is not limited to the following examples.
<Preparation of solvent>
The following compounds were prepared as an oxygen-containing solvent containing an ester bond and glyme.
・MMP: Methyl 3-methoxypropionate ・G1: Ethylene glycol dimethyl ether ・G2: Diethylene glycol dimethyl ether ・G3: Triethylene glycol dimethyl ether ・G4: Tetraethylene glycol dimethyl ether MMP is M3MP manufactured by Nippon Nyukazai Co., Ltd., and G1 is manufactured by Nippon Nyukazai Co., Ltd. DMDG manufactured by Nippon Nyukazai Co., Ltd. was used for G2, DMTG manufactured by Nippon Nyukazai Co., Ltd. was used for G3, and DMTeG manufactured by Nippon Nyukazai Co., Ltd. was used for G4.

<Preparation of electrolyte>
The oxygen-containing solvent and/or glyme containing the above ester bond and the dried and dehydrated product of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) (lithium bis(trifluoromethanesulfonyl)imide manufactured by Tokyo Chemical Industry Co., Ltd.) are listed in Table 1. Each electrolytic solution was prepared by mixing overnight at room temperature with a stirrer at the predetermined molar ratio described in . It was confirmed by thermogravimetry (TG-DTA STA7200RV Hitachi High-Technologies) that a complex of lithium and solvent was formed in each electrolytic solution.

<Performance evaluation>
- Shear viscosity Each of the prepared electrolytic solutions was set in a viscoelasticity measuring device MCR101 (manufactured by Anton Paar), and the shear viscosity was measured at 30° C. and a shear rate of 100 seconds ⁻¹ .

- Ionic conductivity Cellulose (thickness: 25 μm) was impregnated with each of the prepared electrolyte solutions and incorporated into a UFO type two-electrode cell (electrode: SUS-SUS). Impedance was measured at 30° C. using a VSP potentiogalvanostat (manufactured by BioLogic).

- Rate characteristics Cellulose (thickness: 25 μm) was impregnated with each of the prepared electrolyte solutions, and the cells were incorporated into a coin battery (diameter: 20 mm, thickness: 3.2 mm).
A charge/discharge test was conducted at 30° C. under the following charge/discharge conditions using a charge/discharge test device (HJD1010mSM8 manufactured by Hokuto Denko Co., Ltd.) using the following combination of electrodes.
Electrode combination: NMC (positive electrode)/LTO (negative electrode)
Charge/discharge conditions: 1.5-3.0V, 0.08-5.0C
The evaluation is "Excellent" when 50% or more of the initial discharge capacity is retained at 3C, "Good" when 50% or more of the initial discharge capacity is retained at 0.8C, and 50% of the initial discharge capacity at 0.32C. % or more was evaluated as "acceptable", and when less than 50% of the initial discharge capacity was maintained at 0.32C, it was evaluated as "impossible". See FIGS. 3, 4, 7, 8, and 11.

- Oxidation stability The potential when an oxidation current of 0.02 mA/cm ² was passed in the first cycle was measured under the following conditions using the linear sweep voltammetry (LSV) method. The fact that the oxidation current value does not increase up to a higher potential (vs. Li/Li ⁺ ) is an indicator of oxidation stability.
Measuring cell: Polypropylene two-electrode cell Test temperature: 30°C
Potential sweep speed: 0.1 mV/sec Counter electrode and reference electrode: Li
Working electrode: Pt
The evaluation is "excellent" if the potential (vs. Li/Li ⁺ ) when an oxidation current of 0.02 mA/cm ² flows in the first cycle exceeds 5.0 V, and "excellent" if the potential (vs. Li/Li + ) is higher than 5.0 V. A case where the voltage was less than 4.8V was judged as "good", and a case where the voltage was less than 4.8V was judged as "unsatisfactory". See FIGS. 1, 5, and 9.

- Reduction stability The potential was measured by cyclic voltammetry (CV) under the following conditions when a reduction current of 0.05 mA/cm ² was applied in the second cycle. The fact that the reduction current does not increase to lower potentials (vs. Li/Li ⁺ ) is an indicator of reduction stability.
Measuring cell: Polypropylene two-electrode cell Test temperature: 30°C
Potential sweep speed: 0.1 mV/sec Counter electrode and reference electrode: Li
Working electrode: Cu
In the evaluation, when the potential (vs. Li/Li ⁺ ) when a reduction current of 0.05 mA/cm ² flows in the second cycle is less than 0.0V, it is evaluated as "good", and when it is 0.0V or more, it is evaluated as "good". Not possible.” See FIGS. 2, 6, and 10.

- Thermal stability The temperature at which the weight of the sample decreased by 5% was measured by thermogravimetric analysis.
Heating rate: 10℃/min Temperature increasing range: 25-500℃
The results are shown in Tables 1 to 3 and Figures 1 to 11.

From the data in Table 1, the electrolytic solution with the composition G1:M3MP:LiTFSI=1:1:1 contains the oxygen-containing solvent containing ester bonds (M3MP) and glyme (G1) per mole of lithium (Li). It can be seen that the total amount of ether oxygen and carbonyl oxygen (O/Li) is 4 moles, and a high level of well-balanced ionic conductivity, rate characteristics, oxidation stability, reduction stability, and thermal stability are achieved. The other electrolytes in Table 1 consist of either an oxygen-containing solvent containing an ester bond or glyme as a solvent, and have one of the following properties: ionic conductivity, rate characteristics, oxidation stability, reduction stability, and thermal stability. Undesirable data was obtained for one or more items.

From the data in Table 2, the electrolytic solution with the composition G2:M3MP:LiTFSI=1:1:1 contains an oxygen-containing solvent containing an ester bond (M3MP) and glyme (G2) per mole of lithium (Li). It can be seen that the total amount of ether oxygen and carbonyl oxygen (O/Li) is 5 moles, and a high level of well-balanced ionic conductivity, rate characteristics, oxidation stability, reduction stability, and thermal stability are achieved. In addition, the electrolytic solution with the composition G3:M3MP:LiTFSI=1:1:1 contains ether oxygen and carbonyl contained in the oxygen-containing solvent (M3MP) containing an ester bond and glyme (G3) per mole of lithium (Li). It can be seen that the total amount of oxygen (O/Li) is 6 moles, and a high level of ionic conductivity, oxidation stability, reduction stability, and thermal stability are achieved in a well-balanced manner. On the other hand, the electrolytes with composition G1:G2:LiTFSI=1:1:1 and composition G4:LiTFSI=1:1 consist only of glyme as a solvent, and have excellent ionic conductivity, rate characteristics, oxidation stability, reduction stability and Undesirable data was obtained in any one or more of the thermal stability items.

From the data in Table 3, it can be seen that in the electrolytic solution containing an oxygen-containing solvent containing an ester bond and glyme, per 1 mole of lithium (Li), the oxygen-containing solvent containing an ester bond (M3MP) and glyme (G1 to G4) contain The total amount of ether oxygen and carbonyl oxygen (O/Li) is in the range of 3 to 7 moles, ionic conductivity, rate characteristics (no data for G3:M3MP:LiTFSI=1:1:1), oxidative stability, reduction It can be seen that stability and thermal stability are achieved in a well-balanced manner at a high level. On the other hand, the electrolytic solution with the composition G1:G2:LiTFSI=0.7:0.7:1 consisted only of glyme as a solvent, and its oxidation stability decreased.

The electrolytic solution according to the present invention is suitably applied to various rocking chair type secondary batteries, especially lithium ion secondary batteries.

Claims (5)

An electrolytic solution for a secondary battery comprising an alkali metal salt, an oxygen-containing solvent containing an ester bond, and glyme ,
The oxygen-containing solvent containing the ester bond is represented by the following general formula (1),
R ¹ -O-(A-O) _n -CH ₂ CH(R ³ )-COO-R ² (1)
In formula (1), R ¹ and R ² each independently represent a linear or branched alkyl group having 1 to 4 carbon atoms, R ³ represents a methyl group or H , and A each independently represents a carbon number represents an alkylene group of 2 to 4, and n is the average number of added moles of the oxyalkylene group, and represents an integer of 0 to 5;
The grime is represented by the following formula (2),
In formula (2), R ⁴ and R ⁵ each independently represent a linear or branched alkyl group having 1 to 8 carbon atoms, and A 4 ^represents an alkylene group having 2 to 4 carbon atoms. , n is the average number of added moles of the oxyalkylene group, and represents an integer of 1 to 5, and
The total amount of ether oxygen and carbonyl oxygen contained in the oxygen-containing solvent containing an ester bond and the glyme is 3 moles or more and 8 moles or less per mole of the alkali metal of the alkali metal salt. , electrolyte for secondary batteries . The electrolytic solution for a secondary battery according to claim 1, wherein in the formula (1), R ¹ and R ² are both methyl groups, R ³ is H, and n is 0. The electrolytic solution for a secondary battery according to claim 1 or 2, comprising the alkali metal salt, the oxygen-containing solvent containing an ester bond, and the glyme in a substantially 1:1:1 molar ratio. The alkali metal salt may be lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium tetrafluoroborate (LiBF ₄ ), lithium hexafluorophosphate (LiPF ₆ ), or The electrolytic solution for a secondary battery according to any one of claims 1 to 3, which is selected from the group consisting of lithium chlorate (LiClO ₄ ) and lithium bis(pentafluoroethanesulfonyl)imide (LiBETI). A rocking chair type secondary battery comprising a positive electrode, a negative electrode, and the electrolyte for a secondary battery according to any one of claims 1 to 4.

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